PREHEAT OVEN TO 350F/177C. Spray a 9 x 13 inch/22.8 x 33 cm pyrex dish (or Bundt pan) with oil and lightly flour it. To flour, add a tablespoon of flour to sprayed pan, shake to cover evenly, discard remaining flour.
MAKE WET BATTER: Beat eggs and sugar with immersion blender or handheld mixer for 2 minutes in a large bowl until fluffy. Blend in oil and vanilla.
ADD DRY INGREDIENTS TO WET: Put flour mixture of flour, baking powder, salt and cinnamon in a mesh strainer sitting over bowl containing wet batter. Sift/shake dry ingredients through the strainer into the mixture in the bowl below. Blend well with a mixing spoon. Don't over mix. Fold chunks of apple into cake batter. Mix in nuts or raisins if using.
MAKE TOPPING: In small cup, mix together the brown sugar and cinnamon. Sprinkle on top of the cake batter. Note 2 to make crumble topping instead.
BAKE: Pour batter into prepared pan scraping sides of bowl with rubber spatula. Bake apple cake for 35-45 minutes. Check after 35 minutes. If toothpick or tester comes out clean, it's done. If toothpick has batter sticking to it, bake longer until toothpick comes out clean. If using a bundt pan, bake the cake 45-50 minutes or until toothpick/tester comes out clean.
MAKE BOURBON CARAMEL SAUCE: Place sugar and water in a medium saucepan on medium heat. Swirl pan occasionally (don't stir) for 12-15 minutes until a medium to deep amber color appears. Careful that it doesn't burn. Remove from heat. Slowly add the cream while stirring (it will bubble up then settle down). Place back on the burner, add bourbon and butter (and pinch of salt if using unsalted butter). Stir for another minute until silky smooth.
SERVE: Serve warm cake or at room temperature with scoop of vanilla ice cream or whipping cream. Bourbon caramel sauce can be drizzled over the cake before serving or passed on the side.